It is already known to cool a tubular sleeve of thermoplastic material originating from an annular extrusion-blowing die through which the molten material is flowing, the said tube being, in the initial phase of its cooling, in the shape of a cylinder whose diameter is substantially equal to that of the die.
Thus, patents EP-A No. 0,041,803 and EP-A No. 0,077,661 describe devices which have, in common, a closed low cylindrical chamber surrounding the tube as it leaves the die and a blowing ring directing the cooling air at right angles to the tube as it leaves the low chamber. ln the first patent mentioned the device additionally comprises a high chamber placed on the blowing ring and directing the air tangentially to the tube after the latter is inflated. The high chamber described in the second document mentioned has the particular feature of not being joined to the blowing ring and thus enabling an additional induced air current to enter.
Document FR-A No. 1421490 describes a device comprising a rotating ring for blowing air, immediately located at the exit of the die and, intercalated between this ring and the tubular web to be cooled, a collar deflecting the cooling air tangentially to the tube. Document U.S. Pat. No. 3,307,218 describes an apparatus comprising a vertical cylindrical sleeve surrounding the tube when it has reached its greatest diameter and provided with a device making it possible to blow air in a parallel direction to the tube so as to cool it below its deformation temperature.
Patent EP-A No. 0,130,909 describes a device comprising, in its intermediate section, an intensive blowing ring whose current is directed towards the tube and, in its upper section, a high chamber in which the tube is in contact with the current of air directed towards the tube haul-off. In its lower section the device additionally comprises an unclosed low chanber provided with at least one opening.
The devices described in the first two documents mentioned have the disadvantage of being capable of improving the optical properties of the film produced from the tube only in the case of thermoplastic materials which already lead to films whose optical properties are already relatively satisfactory when produced with the aid of a traditional cooling system.
The last document mentioned describes a device which makes it possible to overcome this disadvantage. In practice, however, it has been noted that very slight faults could appear in the films produced from the tube, such as a slight thickness nonuniformity.
The purpose of the process and of the device according to the invention is to improve the thickness uniformity of the tube while maintaining, or even improving, its optical properties.